Road milling, road planing, mining and trenching equipment frequently utilize similar cutting bits and bit assemblies for ripping up old road surfaces, digging trenches for infrastructure or construction purposes, and for mining various minerals. Bit assemblies for these differing applications may use similarly shaped but differently sized bits and bit assemblies. The shanks of road milling bits may approximate ¾ inch in diameter. Road planing bits are larger. Mining bit shanks may very from 1 to 1½ inches in diameter, and trenching bits may be even bigger. Additionally, the length of bit shanks may vary in that road milling bits may not extend completely through a central through bore of a bit holder, while bit shanks used for road planing, trenching and/or mining may extend completely through the bit holder and be retained or fastened on the back side of the bit holder bore.
One such apparatus for holding a cutting bit is shown in U.S. Pat. No. 5,302,005, issued to O'Neill on Apr. 12, 1994.
A bit holder design similar to that shown in FIG. 7 of the O'Neill '005 patent is utilized in the mining industry. The bit holder in the O'Neill '005 design is press fit into the bit block and the dimensions of the outside of the bit holder shank as provided on original equipment machinery are a maximum of 0.005 larger than the corresponding dimensions of the bit block bore. The press or interference fit portions of the bit holder are the cylindrical portions of the outside of the bit holder shank. The central tapered portion of the bit holder shank are less than a seating taper, i.e., less than about 3½ degrees per side. In operation, however, it is understood that typically a multiple set of replacement oversized bit holders are carried in the field with the equipment because of the impacting and abuse that the mining operation imparts on the bit holders and bit blocks of the bit assembly.
It has been discovered that after some use, the press fit on original equipment bit holders becomes loose and tends to pop out of the bit blocks. A first replacement bit holder is utilized that is 0.010 oversize to fit where the original equipment 0.003-0.005 oversize bit holder originally fit. Additionally, 0.015 oversize bit holders and 0.020 oversize bit holders are also carried in the field to replace the 0.010 and 0.015 oversize bit holders when they become loose and also tend to pop out of the bit block bore after extended use. This entire cycle of 0.005, 0.010, 0.015 and 0.020 oversize bit holders may be utilized during the lifespan of a drum or chain assembly on which the bit blocks are mounted.
After reviewing the structure of the bit assemblies shown in the '005 patent, especially those with multiple differing sized, cylindrical upper and lower shank portions, it became apparent to me that the tolerances which need to be met to provide satisfactory surface contact between both the upper and lower cylindrical shank surfaces on the bit holder and their counterparts on the bit block bore are very difficult and costly to achieve, to prevent separation of the holder from the bit block bore in the extreme cutting conditions encountered in road planing, trench digging and mining operations. The solid tapered surfaces also complicate the matching of complementary surfaces between the two parts.
More than likely, press fitting the bit holder in the bit block quickly, or in the field, results in a mis-match of at least one pair of the two pairs of complementary cylindrical surfaces. Any such mis-match would result in voids between at least portions of the adjacent cylindrical surfaces. In use, these voids would aid in movement between the bit holder shank and bit block bore that would increase over time. Eventually, the enlarging movement would deform the adjacent parts until the bit holder would become loose and eject out of the bit block, which is what has happened in the field.
A need has arisen for an improved bit holder, generally of the type used in the O'Neill '005, patent that will remain mounted in its bit block bore during use for an extended period of time.
Another object of the present invention is the provision of an improved bit holder that may be inserted in a bit block bore with a greater interference fit such that it will remain mounted in that bore during abusive use and for an extended period of time, and yet matingly receive the shank of a bit when that the shank extends completely through the central bore of the improved bit holder to prevent deleterious movement between the bit shank and bit holder bore.